Why Would I Use Spanning Trees in My Network?

Redundant links must be factored into even the simplest of topologies to protect against data loss and downtime due to any single point of failure. STP prevents redundant links from forming data loops which would consume all available network bandwidth. STP manages redundant links by keeping them in a blocking state and automatically unblocking them when changes in topology require that they be used. See Spanning Tree Port States for a summary of Spanning Tree port states.

As shown in Redundant Link Causes a Loop in a Non-STP Network, a planned redundant link between Switch 1 and Switch 2 makes it possible for a bridging loop to occur. If Station 1 transmits a multicast or broadcast packet to Station 2 in this scenario, the packet would continue to circulate endlessly between both switching devices. Without Spanning Tree blocking one of the links, there would be nothing at layer 2 to stop this loop from happening and unnecessarily consuming network resources. As administrator, you would be forced to manually disable one of the links between Switch 1 and 2 for the Redundant Link Causes a Loop in a Non-STP Network network to operate.

STP automatically blocks redundant paths, as shown in Loop Avoided When STP Blocks a Duplicate Path. In the event that the primary (unblocked) path fails, STP places the blocked path into service. If the original primary path recovers, the redundant path will once again block and the primary path will be used.